How to use Callgrind to profile specific functions?

Question

Following this, I wrapped my functions with CALLGRIND_xxx_INSTRUMENTATION macros. However, I always get "out of memory".

Here is a simplified version of my program and callgrind would still run out of memory even though I could run callgrind without using the macros.

#include <cstdio>
#include <valgrind/callgrind.h>

void foo(int i)
{
  printf("i=%d\n", i);
}

int main()
{
   for (int i=0; i<1048576; i++)
   {
     CALLGRIND_START_INSTRUMENTATION;
     foo(i);
     CALLGRIND_STOP_INSTRUMENTATION;
   }
}

To run this, "valgrind --tool=callgrind --instr-atstart=no ./foo >foo.out".

Did I do anything wrong? Please help. Thanks!

Answer 1

CALLGRIND_START_INSTRUMENTATION typical use case is to skip instrumenting the application startup code. If you call it in a loop, then this is costly both in memory and in cpu, as callgrind will each time re-instrument the code.

If you are interested in only measuring some functions, you should rather start instrumentation somewhere before the loop, and then use CALLGRIND_TOGGLE_COLLECT before/after the function calls you are interested in. That will use both less cpu and less memory.

If you want to do the above, you should then use the options --instr-atstart=no and --collect-at-start=no. You then start instrumentation at the relevant place in your program (e.g. after the startup/initialisation code). You can then insert calls to CALLGRIND_TOGGLE_COLLECT in the functions you are interested in.

Note that instead of modifying your program to call CALLGRIND_TOGGLE_COLLECT for a bunch of functions, you can also use one or more times the command line option --toggle-collect=<function>